Linear Compressor With Preloaded Spring Piston Rod, and Refrigerating Machine

ABSTRACT

A linear compressor including a piston housing, a compressor piston movable in a reciprocating manner along an axis within the piston housing, a drive mechanism for driving the reciprocating movement, a linkage for connecting the compressor piston to the drive mechanism, and a helical spring operable within the linkage.

The invention relates to a linear compressor comprising a piston housingand a compressor piston movable in a reciprocating manner therein,wherein the compressor piston is connected via a linkage to a drive forthe reciprocal movement, a refrigerating machine and a method forcooling or freezing a product.

It is known how to support and guide a compressor piston in the case ofoil-free linear compressors, using a cushion of coolant in gaseous formwhich streams through micro-openings through a housing wall of a pistonhousing inward to the compressor piston. To maintain the gas pressurebearing effected by the cushion, a continuous feed of gas is necessary,because otherwise the compressor piston comes into contact with thehousing wall, which produces friction and thus leads to wear of thelinear compressor. It follows from DE 695.26 217 T2 that, especially inthe case of gas pressure bearings, touching of the piston and thecylinder should be avoided in the working state, to minimize thefriction and hence the wear of the linear compressor.

In order to keep the consumption of gas as small as possible and toextend the service life of the linear compressor, for a linearcompressor, in particular one with a gas pressure bearing of such atype, the transmission of the linear movement of the linear drive to thepiston by a piston rod must be as free as possible of transverse forces.

The piston rod known from DE 695 26 217 T2 provides a sprung bar or atube, by which forces are transmitted. However, a simple tube or asimple sprung bar is not adequate to provide the necessary combinationof low transverse rigidity, to absorb the unwanted transverse forces,and high axial rigidity for the transmission of a working stroke. Forthis reason, known solutions provide special bar-shaped spring elementshaving two narrow points which serve as bending points and ensure thelowest possible transverse rigidity.

A disadvantage of the known solutions is that, because of the thinbending points, the probability of buckling is comparatively high, evenduring normal operation. It is consequently necessary to develop aseparate specially adapted piston rod for each application situation,i.e. for each compressor variant, so that the sensitive balance betweentransverse and axial rigidity can be maintained as required by thedesign. Over and above this, an attachment of the piston rod to thepiston and to the linear drive is expensive, due to the small diameterof the narrow points.

The solution known from DE 695 26 217 T2 has too low axial rigidity.This can result in loss of performance and disturbances duringoperation, so that under some circumstances the linear compressor worksless efficiently and less reliably.

It is therefore the object of the present invention to specify a linearcompressor or a refrigerating machine in which it is possible to realizea coupling, of the compressor piston to a drive, which is axially rigidbut pliant against transverse forces. A further object is to specify amethod of cooling or freezing a product which works reliably, with ahigh efficiency, and is in a position to cool goods down rapidly andefficiently.

These objects are achieved according to the invention by the linearcompressor, by the refrigerating machine and by the method, as specifiedin the independent claims. Further advantageous embodiments anddevelopments which can each be applied individually or in any desiredcombination, are the subject of the relevant dependent claims.

The linear compressor according to the invention incorporates a pistonhousing and a compressor piston which can move reciprocally along anaxis within the housing, wherein the compressor piston is connected viaa linkage to a drive for the reciprocal movement, wherein the linkagehas a preloaded spring, in particular a helical spring.

The linkage can be formed by one or more piston rods. Several pistonrods can be joined together in parallel and/or in series with eachother.

The linear compressor can be oil-free. In particular, it has a gaspressure bearing by which the compressor piston is located in the pistonhousing without making contact during the reciprocal movement.

The linkage for transmitting a force from the drive to the compressionpiston has a preloaded spring, i.e. a spring which does not change inlength until a prescribed threshold is exceeded for a level of force.

The spring can be manufactured from a metal, in particular steel, oreven from plastics or composite materials. In particular, it is possibleto use fiber-reinforced springs, such as for example carbon-, glass- orpolyaramide-fiber reinforced springs.

It is advantageous if the spring is a close-coiled helical spring, i.e.neighboring turns touch each other, wherein it is advantageous if thesurfaces of neighboring turns lie against each other under pressure.With a spring of this type, a prescribed threshold of force, that is acertain prescribed amount of force, must be exceeded to separate theneighboring turns of the spring from each other. A compression forcewhich is greater than zero can be exerted between two turns of thespring, along the contact area.

With a linkage of this type, it is possible to take up comparativelylarge axial forces, while comparatively small transverse forces can begently cushioned. If the spring is close coiled, then a compressionforce in the longitudinal direction can be taken up by the side surfacesof the turns lying against each other, so that a high longitudinalrigidity is achieved. With a preloaded spring, a high longitudinalrigidity is achieved by the preloading. However, comparatively smalltransverse forces can be absorbed and balanced out as a result of alever effect. Due to this lever effect, and the possibility that a turncan unilaterally lift off the turn adjacent to it, the spring is verypliant in the transverse direction.

Due to the preloading, forces in the longitudinal direction lead to lessdestabilization of the spring in the transverse direction than is thecase with known piston rods.

It is advantageous if the preloading of the spring is greater than anytensile stress which can be created in the linkage by the drive. Thisensures that the spring is not further compressed or stretched, asapplicable, so that a hard coupling is effected between the drive andthe compression piston. In particular, this ensures that the linkagedoes not become an inherently oscillatory system because of the spring.

In a special form of embodiment the linkage has, at each of at least twosections, a preloaded spring. However, it is especially preferred thatthe linkage as such is formed by the spring. By this means, a couplingis effected which is especially pliant in the transverse direction,which can absorb the most diverse modes of oscillation, in particularS-shaped deformations of the linkage.

In a particularly advantageous embodiment of the invention the linkageis screwed, in particular by a section of spring, to the drive and/orthe compressor piston. In particular, due to the particular geometry ofa close-coiled helical spring, there is the possibility of implementingthe connection between the compressor piston and the drive in each caseas a screwed connection, by using the turns at the ends of the helicalspring as a thread, which is more reliable and more cost-effective bycomparison with the known attachment techniques, by gluing or clamping.

The ratio of axial to transverse rigidity for the spring or the linkageaccording to the invention, as applicable, is at least 20:1, inparticular at least 50:1, preferably at least 200:1. Due to itsconstruction, the linkage described has a particularly good ratio ofaxial to transverse rigidity. Consequently the same component can beused for different linear compressors, in particular linear compressorswith different power levels, e.g. 40 Watt, 80 Watt, 120 Watt or 160Watt. Special modification of the linkage for the linear compressor typeconcerned is no longer necessary. This further reduces the manufacturingcosts of the linear compressor.

In an advantageous embodiment of the invention, the compressor piston isguided in the piston housing by means of a housing wall which hasopenings and a gaseous fluid, in particular a coolant, which streamsthrough the openings.

All in all, it is possible with the linear compressor according to theinvention to realize reliable operation with a high efficiency. Powerlosses due to a piston rod which is excessively pliant in thelongitudinal direction are avoided. Also avoided are power losses due toskewing of the compressor piston in the piston housing because ofexcessive rigidity of the piston rod or buckling of the piston rod at anarrow point. Overall, the wear on the linear compressor is thusreduced, and the susceptibility of the linear compressor to faults isfurther reduced.

The refrigerating machine according to the invention, in particular arefrigerator and/or freezer or an air conditioning system for vehicles,incorporates the linear compressor according to the invention. Theadvantageous characteristics of the linear compressor according to theinvention thus carry over to the refrigerating machine, and thus permitan especially reliable, as well as energy-saving, operation of therefrigerating machine which extends the application possibilities of therefrigerating machine, particularly in relation to mobile applicationareas.

The method according to the invention for cooling or freezing a productincorporates the refrigerating machine according to the invention. Dueto the high reliability together with the high efficiency of therefrigerating machine, especially reliable and energy-saving cooling orfreezing of a product is possible. This makes possible especially rapidand cost-effective cooling or freezing of a product.

Further particular advantages or details are explained in more detail byreference to the following drawing, which is not intended to restrictthe invention but merely to illustrate it by examples. Shown inschematic form are:

FIG. 1 a cross-sectional view of a linear compressor according to theinvention,

FIG. 2 a cross-sectional view of a section of another linear compressoraccording to the invention,

FIG. 3 a perspective view of a linear compressor according to theinvention, and

FIG. 4 a refrigerating machine according to the invention.

FIG. 1 shows a longitudinal section through a linear compressor 1according to the invention, with a compressor piston 4, which hasa′drive 6 and a linkage 5, for connecting the compressor piston 4 withthe drive 6, within a housing wall 14 which has openings 12 throughwhich fluid streams to provide a bearing. The linkage 5 which takes theform of a piston rod is formed by a close-coiled helical spring 7 whichis subject to preloading. The preloading amounts to some 5 to 50Newtons, i.e. at least 5 to 50 Newtons must be applied to the linkage 5in its axial direction to separate neighboring turns 16 from each other.Due to the lever effect however, only 0.14 to 1.4 Newtons are necessaryin the transverse direction with the leverage shown here to effect thelifting of one turn off its neighboring turn. The compressor piston 4moves reciprocally along an axis 3 in the housing wall 14, which is inthe form of a cylindrical sleeve. The bearing for the piston 4 iseffected with the help of a fluid 17, which flows between the housingwall 14 and the piston housing 2 and through the openings 12. The fluidstream forms a gas cushion, so that a gas pressure bearing is created.

FIG. 2 shows a longitudinal section through a section of another linearcompressor 1 according to the invention, wherein the linkage 5 istubular in form and on a first section 8 and on a second section 9 arearranged close-coiled preloaded springs. The two sections 8, 9 permitbending of the linkage 5 in these regions, and thus an especially lowtransverse rigidity.

FIG. 3 shows a perspective view of a linear compressor 1 according tothe invention, with a drive 6, a piston housing 2, wherein a compressorpiston (not shown) is driven by the drive 6 with the help of a linkage5.

The compression pressure is 9 to 10 bar. The mass of the compressorpiston 6 is 50 g. The working frequency is somewhat below 50 Hz. Thestroke of the compressor piston is about 20 mm and the volume which iscompressed is from 1 to 9 cm³.

FIG. 4 shows the refrigerating machine 13 according to the invention,with the linear compressor 1 according to the invention and goods 15which are being cooled. Especially reliable, rapid and energy savingcooling of the goods 15 is made possible by the high reliability andhigh efficiency of the linear compressor 1 according to the invention.

The invention relates to a linear compressor 1 comprising a pistonhousing 2 and a compressor piston 4 movable in a reciprocating mannertherein along an axis 3, wherein the compressor piston 4 is connectedvia a linkage 5 to a drive 6 for the reciprocating movement, wherein thelinkage 5 has a preloaded spring 7, together with a refrigeratingmachine and a method of cooling or freezing a product comprising thelinear compressor 1 according to the invention. The invention isdistinguished by the fact that, on account of the type of constructionof the preloaded spring, an especially good ratio of axial to transverserigidity of the coupling between drive 6 and compressor piston 4 can beachieved and therefore one and the same component can be used forvarious linear compressors having different output stages. This reducesnot only the variance in the piston rods but also the number offastening parts at the piston and the drive ends.

LIST OF REFERENCE CHARACTERS

-   1 Linear compressor-   2 Piston housing-   3 Axis-   4 Compressor piston-   5 Linkage-   6 Drive-   7 (Helical) spring-   8 First section-   9 Second section-   10 First section of spring-   11 Second section of spring-   12 Openings-   13 Refrigerating machine-   14 Housing wall-   15 Goods-   16 Turns-   17 Fluid

1-10. (canceled)
 11. A linear compressor comprising: a piston housing; acompressor piston movable in a reciprocating manner along an axis withinthe piston housing; a drive mechanism that drives the reciprocatingmovement; a linkage that connects the compressor piston to the drivemechanism; and a helical spring within the linkage.
 12. The linearcompressor according to claim 11, wherein the spring is a close coiledhelical spring.
 13. The linear compressor according to claim 11, whereinthe preloading of the spring is greater than any tensile stress whichcan be produced in the linkage by the drive mechanism.
 14. The linearcompressor according to claim 11, wherein a preloaded spring is at eachof two sections of the linkage.
 15. The linear compressor according toclaim 11, wherein the linkage comprises the spring.
 16. The linearcompressor according to claim 11, wherein a section of the linkage witha section of spring has a screwed fixing the linkage to one of the drivemechanism and the compressor piston.
 17. The linear compressor accordingto claim 11, wherein the ratio of axial to transverse rigidity for thespring is at least 20:1, in particular at least 50:1, preferably atleast 200:1.
 18. The linear compressor according to claim 11, whereinthe compressor piston is guided in the piston housing by a housing wallwhich has openings and a coolant, which enters the housing through theopenings.
 19. A refrigerating machine including one of a refrigerator, afreezer and an air conditioning system with the refrigerating machinecomprising: a linear compressor including a piston housing; a compressorpiston movable in a reciprocating manner along an axis within the pistonhousing; a drive mechanism for driving the reciprocating movement; alinkage that connects the compressor piston to the drive mechanism; anda helical spring within the linkage.
 20. A method for cooling orfreezing a product by the use of a refrigerating machine, the methodcomprising: providing a refrigerating machine having: a linearcompressor including a piston housing; a compressor piston movable in areciprocating manner along an axis within the piston housing; a drivemechanism that drives the reciprocating movement; a linkage thatconnects the compressor piston to the drive mechanism; and a helicalspring within the linkage, and exposing the product to the atmospherewithin the refrigerating machine for a predetermined time duration.